1. Field of the Invention
The present invention relates to an electric power unit for supplying the output power of a battery to a load circuit through a chopper-controlled nonisolated DC-DC converter (hereinafter referred to simply as "DC-DC converter") and, more particularly, to an electric power unit capable of supplying the energy stored in a battery to a load circuit at the least possible loss of energy.
2. Description of the Prior Art
Recent cordless electronic appliances, such as portable computers, portable word processors, portable VTRs and portable video cameras, are provided with a built-in electric power unit comprising a battery, such as a nickel-cadmium secondary battery, and a DC-DC converter for converting and stabilizing the output voltage of the battery. Generally, the battery comprises a plurality of cells connected in series, and the output voltage of the battery is converted into a voltage required by the load circuit by the DC-DC converter.
In designing a battery-powered electronic appliance provided with an electric power unit, it is a very important technical problem to extend the continuous duty time, namely, a time period in which the battery-powered electronic appliance is able to operate without recharging the battery or without changing the battery, under limited conditions for the dimensions and weight of the battery-powered electronic appliance. Accordingly, many studies have been made to increase the energy density of the battery and to reduce the power consumption of the circuit. As is well known, power loss in the chopper-controlled DC-DC converter, i.e., a switching regulator, is far less than that in the conventional series regulator and hence the chopper-controlled DC-DC converter is recently applied to many electronic appliances. However, the power loss even in the chopper-controlled nonisolated DC-DC converter is in the range of 20 to 25%, which is significantly large in view of further reducing the power consumption of the electronic appliance, and the electric power unit itself has a significant cause of reducing the continuous duty time of the battery-powered electronic appliance.
If the number of series-connected cells of an electric power supply is determined selectively so as to meet the required voltage of the load circuit, the DC-DC converter may be omitted. Some battery-powered electronic appliances are provided with such an electric power unit. In designing such an electric power unit for a battery-powered electronic appliance, the number of series-connected cells of the battery is determined so that the sum of the voltage of the cells in the stable discharge period in the discharge characteristic curve, i.e., the nominal voltage, coincides with the required voltage of the load circuit of the battery-powered electronic appliance. The output voltage of the battery decreases gradually as the battery is discharged, and warning is given to prompt the operator to stop the use of the appliance, or the electric power unit is disconnected automatically to prevent the malfunction of the load circuit, upon the drop of the output voltage below a threshold value. That is, the final discharge voltage of the battery is set for a voltage slightly higher than a voltage below which the load circuit is unable to function normally, and the discharge of the battery is stopped upon the decrease of the output voltage of the battery below the final discharge voltage. As is well known, the output voltage of the battery, in general, gradually decreases as the battery is discharged. Even if the discharge of the battery is stopped upon the decrease of the output voltage below a certain final discharge voltage, the battery is not completely exhausted; a considerable amount of energy still remains in the battery. Accordingly, the electric power unit not employing a DC-DC converter is unable to use the residual energy remaining in the battery after the output voltage has dropped below the final discharge voltage, so that the continuous duty time of the electronic appliance is diminished.